Hi PGMom! Welcome to 30-something's! I don't think I've read anything specifically about that, but I'm guessing that sugary carbonated beverages could hinder your appetite. As far as not gaining anything, if your Dr. isn't concerned, I wouldn't worry about it. There's a good chance that if you're overweight, your body might just be using your stored fat, ya' know?
By the way, I am by no means a professional, and all this is just my opinion! Please speak with your Dr. if you have serious concerns!
OK, that being said, I just had a baby 2 1/2 wks. ago, and I ended up only gaining 9 lbs. My baby was 9.4 lbs. My midwife was not concerned that i was losing weight toward the end of my PG, because the baby was fine.
Maybe if you tried to cut down on the soda, and drink some more water, you'll see a difference. Please keep us posted!!